Why are my sim cards not reading

hi there all,

i have an iphone 5c thats not reading simcards. the problem appears from nothing and i have updated my device to latest os in latest itunes. it can flash baseband and imei shows. no water damage appears inside. and i have checked the switch inside the sim reader it is closing and opening correctly. very sometimes if my device is in my pocket and it shows shorty sim locked (enter pin) then 4g but then short after that its disappears again that annoying no sim issue. i have a brand new iphone se but i want to use this 5c because i want keep my se brand new and this 5c can get defective if that happens.